Most of the kernel code is in the drivers, the general purpose subsystems (VFS, I/O scheduler, task schedulers, memory management etc.) are a small fraction of

Most of the kernel code is in the drivers, the general purpose subsystems (VFS, I/O scheduler, task schedulers, memory management etc.) are a small fraction of those 25 million LOC and largely independent of each other so it is not that hard to build some understanding of them. Some ways you can start: - Here is start_kernel(), the kernel entry point after booting up and handling the lowest level stuff in asm: https://github.com/torvalds/linux/blob/v4.19/init/main.c#L531 https://github.com/torvalds/linux/blob/v4.19/init/main.c#L53... - grep for SYSCALL_DEFINE to find definitions of syscalls, e.g. this is open(): https://github.com/torvalds/linux/blob/v4.19/fs/open.c#L1076 https://github.com/torvalds/linux/blob/v4.19/fs/open.c#L1076 (understanding how the I/O and networking system calls work is quite helpful for application developers, even if you work in node.js, python or another high level language) - this is the struct that represents each process in the system, you can pick some interesting field and search for where it is used and where updated: https://github.com/torvalds/linux/blob/v4.19/include/linux/sched.h#L593 https://github.com/torvalds/linux/blob/v4.19/include/linux/s... Finally, the linux-insides book is pretty helpful: https://0xax.gitbooks.io/linux-insides/ https://0xax.gitbooks.io/linux-insides/